Comparisons of techniques for measuring shortwave absorption and black carbon content of aerosols from biomass burning in Brazil

Six methods for measuring the shortwave absorption and/or black carbon (BC) content of aerosols from biomass burning were compared during the Smoke, Clouds, and Radiation-Brazil (SCAR-B) experiment. The methods were the optical extinction cell (OEC), integrating plate (IP), optical reflectance (OR), particle soot/absorption photometer (PSAP), thermal evolution (TE), and remote sensing (RS). Comparisons were made for individual smoke plumes and for regional hazes dominated by smoke. Taking the OEC as a primary standard, measurements of the absorption coefficient (sigma(a)) showed that the OR method had the lowest uncertainty (17%) in sigma(a). The other optical methods had uncertainties ranging from 20 to 40%. However, with sufficient sample size, the values of sigma(a) derived from the optical methods converged to within 20% of each other. For biomass burning aerosols in regional hazes over Brazil, this led to systematic differences of +/-0.02 in the values of the single-scattering albedo derived from the various in situ techniques. It was found also that the BC content of the aerosol and sigma(a) were poorly correlated. This is likely due to a large uncertainty in the BC content of the aerosol measured by TE, and/or a high variability in the mass absorption efficiency of BC in biomass burning aerosol. Hence there is a high uncertainty in inferring sigma(a) from the BC content of smoke aerosol.